Optically controlled electrical-switch device based upon carbon nanotubes and electrical-switch system using the switch device

ABSTRACT

Described herein is an optically controlled electrical-switch device which includes a first current-conduction terminal and a second current-conduction terminal, and a carbon nanotube connected between the first and the second current-conduction terminals, the carbon nanotube being designed to be impinged upon by electromagnetic radiation and having an electrical conductivity that can be varied by varying the polarization of the electromagnetic radiation incident thereon. In particular, the carbon nanotube may for example, in given conditions of electrical biasing, present a high electrical conductivity when it is impinged upon by electromagnetic radiation having a given wavelength and a polarization substantially parallel to the axis of the carbon nanotube itself, and a reduced electrical conductivity when it is impinged upon by electromagnetic radiation having a given wavelength and a polarization substantially orthogonal to the axis of the carbon nanotube itself.

PRIORITY CLAIM

The present application is a Divisional of copending U.S. patentapplication Ser. No. 10/863,635, filed Jun. 7, 2004; which claims thebenefit of Italian Patent Application No. TO2003A000425, filed Jun. 6,2004; all of the foregoing applications are incorporated herein byreference in their entireties.

BACKGROUND

Embodiments of the present invention relate to an optically controlledelectrical-switch device based upon carbon nanotubes and to anelectrical-switch system using the switch device.

In the last few years, the considerable success of CMOS technology hasbeen determined fundamentally by the possibility of constantly reducingthe dimensions of electronic devices. In fact, this technology followsthe so-called Moore's law, according to which the number of transistorsthat can be obtained on an integrated circuit and, consequently, thespeed of calculation should double in a time range of between 18 and 24months.

However, it is a common conviction that conventional siliconmicro-electronics cannot continue indefinitely to follow this law in sofar as sooner or later physical limits that prevent current circuitsfrom functioning in a reliable way at nanometric dimensions willcertainly be reached, while at the same time an exponential increase inproduction costs will render any further increase in the levels ofintegration prohibitive. By increasing the density of the electronicdevices on a chip, in fact, phenomena such as the need to dissipate theheat generated by such dense circuits and the transition from theclassic behavior to the quantum behavior of charge carriers willconsiderably slow down progress.

In particular, thanks to the use of lithographic techniques, there havecurrently been reached dimensions of the order of 100 nm.Notwithstanding the rapid progress achieved in the current process ofscale integration, current technology is difficult to scale furtherbelow these critical dimensions. In fact, once the critical dimensionshave been reached, the small electrical currents that carry theinformation are transferred uncontrollably from one device to the other.In particular, when quantum effects start to become important, thetransistors tend to lose the electrons that represent the information,so that it becomes difficult to maintain them in the original state. Itis envisaged that, below the dimensions indicated of 100 nm, thesedifficulties are likely to become important.

The need to solve the above problems has forced research in thedirection of the study of new technologies based upon the use of organicmaterials that can replace, altogether or in part, silicon in theconstruction of electronic devices.

Molecular electronics has the potentiality for overcoming the limits ofsilicon technology in so far as it is possible to fabricate singlemolecule devices that organize themselves in parallel by means ofself-assembly techniques, which are also economically advantageous.

The need has thus arisen to explore the possibility of passing fromcurrent assembly technologies of the top-down type, whereby it ispossible to reach the desired dimensions with successive removals of amacroscopic amount of a material, to technologies of the bottom-up type,whereby it is possible to make, and subsequently assemble, nanometriccomponents starting from individual atoms or molecules, i.e., ones inwhich the devices involved in handling and retaining the data aremolecules arranged and interconnected so as to form a circuit.

Amongst the different molecular structures studied, carbon nanotubes(CNTS) have aroused an enormous interest owing to their extraordinaryphysical properties. For a detailed treatment, see for example thearticle “Carbon Nanotube-Based Nonvolatile Random Access Memory forMolecular Computing”, Science, Volume 289 (5476), Jul. 7, 2000, 94-97.

It is known the property of carbon atoms to organize themselves intodifferent structures, giving rise to materials of different forms. Infact, diamond is made up of carbon atoms organized in tetrahedrons,while graphite is made up of carbon atoms organized in planarstructures. These two allotropic forms, albeit arising from the sametype of atoms, exhibit structural properties (hardness, elasticity,friction) and functional properties (electrical conductivity, color,etc.) that are very different and frequently opposite.

The structural characteristics, such as hardness and refractoriness, ofgraphite and diamond render it difficult to implement, at nanometricscales, an on-device approach of the top-down type. Instead, an approachof the bottom-up type is made possible by the use of another allotropicform of carbon, namely fullerene.

Belonging to the family of the fullerenes is C₆₀, also known asbuckyball, which has a molecular structure having the shape of apolyhedral cage, made up of pentagons and hexagons. The structures offullerenes, which develop in the form of long cylinders, rather than inthe form of spheres, are called nanotubes. Their length (severalmicrons) can be thousands of times larger than their diameter Oust a fewnanometers). Furthermore, using known techniques of molecular synthesis,there have been observed, in the laboratory, single-walled cylindricalstructures, or single-walled nanotubes (SWNTs), having a diameter of 1-2nm, and multiple-walled structures, or multiple-walled nanotubes(MWNTs), i.e., ones formed by coaxial cylinders, with diameters of sometens of nanometers.

Carbon nanotubes are organic molecules made up of a number of carbonatoms interconnected in a cylindrical structure, which are characterizedby a low weight and have exceptional elastic properties that render themextremely hard but also capable of undergoing large deformations withoutbreaking. Thanks to their exceptional mechanical properties and to theircapacity of conducting electrical charges, carbon nanotubes, in so faras they can be configured both as conductors and as semiconductors, aresuited for forming the components of a new class of nanometricelectronic devices. In particular, they are believed to play a primaryrole in the development of molecular electronics on account of the factthat, thanks to their lateral dimensions of the order of the nanometerand their electrical conduction properties, they behave as quantumconductors of nanometric dimensions (“quantum nanowires”).

Carbon nanotubes have different shapes that can be described by avector, referred to as chiral vector C, as illustrated in FIG. 1.

In particular, in geometrical terms, a carbon nanotube can be obtainedfrom a sheet of graphite, by “cutting” it along lines (dashed lines inFIG. 1) perpendicular to the chiral vector and by “rolling” it in thedirection of the chiral vector itself. In this way, a cylinder ofdiameter d=|C|/π is formed.

The chiral vector consequently defines the type of winding to which theindividual sheet of graphite is subjected in order to give rise to aparticular carbon nanotube. When the sheet of graphite is wound to formthe cylindrical part of the carbon nanotube, the ends of the chiralvector are joined. The chiral vector hence represents a circumference ofthe circular section of the nanotube.

The chiral vector C can be set in relation to two unit vectors a₁ and a₂that define the lattice of the planes in the graphite, by means of twoindices n and m, according to the following equation:

C=n·a ₁ +m·a ₂

Linked to the indices n and m are an angle φ, referred to as chiralangle, and

$\varphi = {\arccos \lfloor {\sqrt{3}{( {n + m} )/2}\sqrt{( {n^{2} + m^{2} + {nm}} )}} \rfloor}$$d = {\frac{a}{\pi}\sqrt{n^{2} + m^{2} + {nm}}}$

the diameter d of the carbon nanotube according to the followingequations:

The values of the indices n and m define the chirality of the carbonnanotube, which is the state of the carbon nanotube itself, whichdiffers according to the way in which the hexagons of the graphitearrange themselves in forming the cylindrical structure. The chiralityof a carbon nanotube is thus given by the pair of integer indices (n, m)and determines the structural characteristics and, consequently, theelectrical conduction properties of a carbon nanotube. In particular, inrelation to the structure, nanotubes that have the indices n and mequal, i.e., nanotubes (n, n), are referred to as armchair nanotubes onaccount of the arrangement of the hexagons of graphite with respect tothe axis of the carbon nanotube itself; nanotubes in which one of thetwo indices is zero (n, 0) are referred to as zigzag nanotubes;nanotubes for which the relation m=0 or else n=m is valid are referredto as achiral nanotubes; while nanotubes with different indices are ingeneral referred to as chiral nanotubes.

The chirality conditions the conductance of the carbon nanotube, itsdensity, its lattice structure, and other properties. The chiral indicescan, in principle, be obtained experimentally, by measuring the chiralangle φ and the diameter d of the carbon nanotube with a transmissionelectron microscope (TEM) or with a scanning tunneling microscope (STM).

Furthermore, according to their chirality the nanotubes can be metallicnanotubes or semiconductor nanotubes. In fact, nanotubes whose chiralityindices satisfy the following relation:

n−m=3·I I=0, 1, 2,

are metallic and, hence, conductors; all the others have a nonzerobandgap and, consequently, behave as semiconductors. Armchair nanotubesare metallic.

The fundamental bandgap of a semiconductor carbon nanotube depends uponthe diameter d of the carbon nanotube, on the basis of the followingrelation:

E _(gap)=2y ₀ a _(cc) /d

where y₀ is the binding energy of the carbon atoms, and a_(cc) is thedistance between two neighboring carbon atoms.

Consequently, by appropriately modifying the chirality of the carbonnanotube and, hence, its diameter, it is possible to modulate itsbandgap. The two different geometrical structures of the molecule (i.e.,the initial one and the modified one) can thus represent two stablestates.

Carbon nanotubes can be produced in macroscopic amounts using differenttechniques: laser ablation, arc discharge, or else chemical-vapordeposition (CVD). For a more detailed treatment as regards the lattertechnique see for example H. M. Cheng et al., Appl. Phys. Lett. 72, 3282(1998).

In particular, the latter technique is compatible with the methods usedin the micro-electronics industry and enables nanotubes to be grown onsubstrate. Using the various techniques, it has been found that thecarbon nanotube that can be produced in the largest quantity is thecarbon nanotube (10, 10).

As has been said, carbon nanotubes constitute a way for responding tothe need to reduce the dimensions of devices in integrated circuits. Infact, by means of the versatile molecules, the path has been opened tothe construction of molecular transistors.

In the last few years, different configurations of field-effecttransistors have been proposed that use carbon nanotubes withsemiconductor properties as channels for the transport of electricalcharges. Some research groups (R. Martel et al. of the IBM ResearchDivision, Chongwu Zhou et al. of the University of Southern California,etc.) have obtained a so-called “back-gate” configuration, illustrated,in schematic cross section, in FIG. 2, in which the substrate functionsas gate of the device. The configuration renders, however, impossiblethe integration of a high number of devices on one and the same chip. Infact, in this case it would be necessary to apply the same gate voltageto all the transistors on the chip. For a more detailed treatment of thesubject, see R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph.Avouris, App. Phys. Lett. 73, 2447 (1998) and C. Zhou, J. Kong, E.Yenilmez, H. Dai, Science, 290,1552 (2000).

Subsequently, in 1998 researchers of the Dekker group of DelftUniversity of Technology developed carbon-nanotube field-effecttransistors (CNT-FETs) using an innovative configuration referred to as“local gate” configuration, illustrated in FIG. 3, which enablesintegration of a large number of devices on a single chip. For a moredetailed treatment of the subject see A. Bachtold, P. Hadley, T.Nakanishi, C. Dekker, Science, 294, 1317(2001).

Broadly speaking, in the CNT-FETs the channel is constituted by a carbonnanotube functioning at room temperature, and the local gate isinsulated from the carbon nanotube by means of an oxide layer of just afew nanometers in thickness. In particular, two gold electrodes weredeposited, which function as source and drain, on a silicon oxidesubstrate grown on silicon, which functions as gate, and these twoelectrodes were connected with a single-walled nanotube (SWNT), whichfunctions as channel. The current-voltage characteristic of thisthree-terminal device was measured and it was verified that it respectedthe characteristic of a field-effect transistor.

In particular, FIG. 3 illustrates a schematic cross section of thelocal-gate CNT-FET The gate in the device is constituted by an aluminiumwire with an insulating layer of Al₂O₃, which separates it from thecarbon nanotube. The semiconductor carbon nanotube is set in electricalcontact with the two gold electrodes. The thickness of the Al₂O₃ layer(a few nanometers) is much smaller than the separation between theelectrodes (˜100 nm). This determines an excellent capacitive couplingbetween the gate and the carbon nanotube, the consequences of whichresult in a gain in voltage greater than 10 and in a wide range of theoutput signal. It is possible to design various aluminium local gates insuch a way that each will address a different CNT-FET.

Formation of circuits comprising CNT-FETs is articulated in threefundamental steps. In the first step, the gate is obtained bydelineating the aluminium pattern via electron-beam lithography (e-beamlithography) on an oxidized silicon wafer. The layer of insulatingmaterial is represented by an oxide grown by exposing the specimen toair. The thickness of the layer cannot be determined with greatprecision but is in the order of a few nanometers. The second stepconsists, instead, in dispersing single-walled nanotubes (SWNTs),previously produced by laser ablation, on the wafer starting from asuspension of dichloroethane. The nanotubes of appropriate diameter (˜1nm) are selected and positioned on the top part of the aluminium gate.An alternative technique envisages the growth in situ of the nanotubesusing the chemical-vapor-deposition (CVD) technique assisted byorienting electrical fields (for a detailed treatment of the subject seeY. Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E.Yenilmez, J. Kong, H. Dai, App. Phys. Lett. 79, 3155 (2001)). Finally,the last step consists in the formation of electrodes andinterconnections via electron-beam lithography, evaporating golddirectly on the carbon nanotube without intermediate adhesion layers.

The CNT-FET illustrated in FIG. 3 is a p-type device functioning byenrichment since it is possible to obtain a marked modulation of thecurrent through the FET by applying a small negative voltage to thegate. Furthermore, by acting on the gate voltage it is possible to varythe carrier concentration of the carbon nanotube, up to reversing itspolarity from the p-type regime to the n-type regime.

FIG. 4 illustrates the current/source-drain voltage characteristics(I/V_(sd)) of the transistor measured at room temperature for differentvalues of the gate voltage (V_(g)). As may be noted, the trend of thecurves is typical of traditional FETs with finite values of the current,when the gate voltage is negative and smaller than the threshold voltageV_(t) (V_(t)˜−1.0 V).

The same research group mentioned above proposed, moreover, thepossibility of making some elementary logic circuits based uponCNT-FETs. The applications (OR gates, AND gates, NOT gates, SRAMS) wereobtained using the resistor-transistor logic scheme and forming theCNT-FETs on the same chip.

FIG. 5 gives the input-output transfer characteristics of a logicinverter with carbon-nanotube FETs and a pull-up resistor on the outsidethe chip with the value of 100 MΩ. The researchers identified thevoltage of ˜1.5 V as an appropriate value for logic applications (logic0=0 V, logic 1=−1.5 V).

When the input of the logic inverter assumes the logic value “1”(V_(in)=−1.5 V), the negative gate voltage induces a movement ofelectron holes in the carbon nanotube giving it a resistance that isconsiderably smaller than the pull-up resistance, and this pushes theoutput to the logic value “0” (V_(out)=0 V). When the input assumesinstead the logic value “0” (V_(in)=0 V), the carbon nanotube is notconductive and the output assumes the logic value “1” (V_(out)=−1.5 V).

Recently, some studies have demonstrated the possibility of makingmolecular devices with non-electrical control using carbon nanotubes. Inparticular, an example of optically controlled molecular device basedupon carbon nanotubes has been proposed by a research group of theUniversity of California. For a detailed treatment of the subject see D.W. Steuerman, A. Star, R. Narizzano, H. Choi, R. S. Ries, C. Nicolini,J. F. Stoddart, J. R. Heath, J. Phys. Chem. B 106, 3124 (2002).

The above work investigated the interactions between single-walledcarbon nanotubes (SWNTs) and two types of polymers:

-   -   PmPV        (poly{(m-phenylene-vinylene)-co-[(2,5-dioctyloxy-p-phenylene)vinylene]}),        and    -   PPyPV        (poly{(2,6-pyridinylene-vinylene)-co-[(2,5-dioctyloxy-p-phenylene)vinylene]}).

The two polymers indicated above are structurally similar and, insolution, are characterized by the same absorption spectrum with a peakin the proximity of 420 nm. The most significant difference is linked tothe fact that the PPyPV is a base and, for this reason, is protonated bythe HCl present in the solution in which the polymer is dissolved. Theinteraction with the carbon nanotubes favors the protonation process.The device obtained consists simply of two metal electrodes, betweenwhich are arranged the polymer/CNT blends (polymer-wrapped SWNTS),deposited by spin coating.

The above polymers and the carbon nanotubes are in electrical contact:in this way, it is possible to use radiation of appropriate wavelengthfor the purpose of modulating the electrical conductivity of the ropesof carbon nanotubes. The researchers carried out a series ofmeasurements at different wavelengths for devices containing just ropesof CNTs, PmPV-wrapped/CNTs and PPyPV-wrapped/CNTs, and the currentresponses obtained are illustrated in FIGS. 6 a, 6 b, and 6 c,respectively.

In particular, as emerges clearly from FIG. 6 a, in the devicescontaining only CNTs no type of optically modulated response isdetected. FIGS. 6 b and 6 c show, instead, the responses of thePmPV-wrapped/CNTs device with negative and positive electrical biasing,respectively. The variation of the output response is the same whateverthe electrical biasing and is approximately 15-20% of the total current.In particular, there appears a photo-amplification of the current forpositive biasing, i.e., the intensity of the total current in the device(dark current plus photogenerated current) increases, in absolute value,when the light at input is ON; on the other hand, for negative biasingthere is a photorectification; consequently, in conditions ofillumination the total current diminishes.

FIG. 7 illustrates, instead, the current response of a device withPPyPV-wrapped/CNTs. As may be noted, no effect is observed for negativebiasing, while for positive biasing the effect of photo-amplification issubstantially greater as compared to the previous case illustrated inFIGS. 6 a, 6 b, and 6 c.

Using structures of this type, it is possible to make opticallycontrolled electrical switches. In fact, as is evident from FIGS. 6 band 6 c, to a switching of the optical signal at input (light ON—lightOFF) there corresponds a switching of the electrical signal at output.

The fundamental limits of the above devices are linked basically to thecomplexity of their implementation in integrated technology on accountof the technique used for depositing the polymer/CNT blend. Thespin-coating technique, in fact, renders it extremely difficult todefine on the chip delimited areas on which to deposit the solution. Thedelineation of these areas would require a further process of controlledremoval of the “spin-coated” solution, to be appropriately defined onthe basis of the properties of the starting solution. Furthermore, spincoating does not enable a good control of the uniformity of thickness ofthe deposited film to be obtained.

Another important embodiment is linked to the low temperatures at whichthe devices have been made and tested (T_(op)=4 K). From the literature,in fact, their behavior at room temperature is not known.

The purpose of embodiments of the present invention is to provide anoptically controlled electrical-switch device that will enable thedrawbacks of the known devices described above to be overcome at leastin part.

SUMMARY

According to an embodiment of the invention an optically controlledelectrical-switch device is provided.

According to an embodiment of the invention, there is provided anelectrical-switch system.

According to an embodiment of the invention, there is further providedan optical-control method of an electrical switch device.

According to an embodiment of the invention, there is provided afabrication process of an optically controlled electrical-switch device.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention there is nowdescribed an embodiment, provided purely by way of non-limiting exampleand with reference to the annexed drawings, in which:

FIG. 1 shows the geometrical characterization of a carbon nanotube;

FIG. 2 shows a schematic cross section of a carbon-nanotube FET in theback-gate configuration;

FIG. 3 shows a schematic cross section of a local-gate CNT-FET;

FIG. 4 shows voltage/current characteristics measured at roomtemperature for different gate-voltage values of a CNT-FET;

FIG. 5 shows the transfer characteristic of a logic inverter withCNT-FETs;

FIGS. 6 a, 6 b, and 6 c show the current responses, respectively, of anoptically controlled device obtained using CNTs, of an opticallycontrolled device obtained with PmPV-wrapped/CNTs with negativeelectrical biasing, and of an optically controlled device obtained withPmPV-wrapped/CNTs with positive electrical biasing, in the presence andin the absence of incident light;

FIG. 7 shows the current response of an optically controlled deviceobtained with PPyPV-wrapped/CNTs in the presence and in the absence ofincident light and as the wavelength of the incident light varies;

FIGS. 8 a, 8 b, and 8 c show, respectively, the model of a single-walledcarbon nanotube, namely, a zigzag (4,0) nanotube, the corresponding bandstructure, and the corresponding electron density of states;

FIG. 9 shows a table of the rules of selection for the absorption of theincident light upon a carbon nanotube;

FIG. 10 shows the absorption spectrum of a carbon nanotube for lightpolarized in a direction parallel to and perpendicular to the axis of acarbon nanotube;

FIG. 11 shows a possible implementation on silicon of an opticallycontrolled electrical-switch device according to an embodiment of thepresent invention;

FIG. 12 shows the circuit diagram of a logic inverter with active loadusing an optically controlled electrical-switch device according to anembodiment of the invention;

FIG. 13 shows a possible implementation on silicon of the logic inverterof FIG. 12;

FIG. 14 shows a different possible use of an optically controlledelectrical-switch device according to an embodiment of the invention;and

FIGS. 15 and 16 show the diagrams of two electro-optical switches,respectively, according to an embodiment of the present invention andaccording to the known art.

An innovative idea underlying embodiments of the present invention drawsorigins from different theoretical studies regarding the electronic andstructural properties of carbon nanotubes that have highlighted howthese molecular structures have the property of optical dichroism.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

In general terms, optical dichroism is the property of a means to absorbin a different way the two components of polarization of light thatpropagates in an anisotropic means. In particular, in an axisymmetricalmeans the two components of polarization of light referred to are theone parallel to and the one orthogonal to the optical axis.

In practice, a dichroic means behaves like a polarizing sheet: onedirection of polarization is totally or partially transmitted, the oneorthogonal thereto is absorbed.

The purpose of embodiments of the present invention is thus to exploitthe property of optical dichroism of nanotubes in order to provide anelectrical-switch device capable of switching between the two operatingstates, open and closed, by simply varying the polarization of the lightincident upon the device itself.

The property of optical dichroism has been observed both in chiralnanotubes and in achiral nanotubes. Furthermore, different experimentalstudies have demonstrated that films of aligned nanotubes arebirefringent on account of the difference in the dielectric functionsfor light polarized in a direction orthogonal to and a directionparallel to the axis of the nanotubes itself. In particular, a work hasrecently been published (I. Bozovic, N. Bozovic, M. Damnjanovic, Phys.Rev. B 62, 6971 (2000)) regarding the effect of optical dichroism in anindividual carbon nanotube. The effect is linked to the specific rulesof selection of the linear group for interband transitions. In Bozovic'swork a single-walled carbon nanotube is considered, namely a zigzag (4,0) nanotube with translational lattice pitch along the axis of thecarbon nanotube a=4.26 Å. The spatial-symmetry group of the carbonnanotube is the linear group L8₄/mcm; this code is connected to the setof different symmetry properties of the carbon nanotube having acylindrical geometry and refers to the symmetries of rotation about thevertical axis of the tube (8 ₄) and to the symmetries of reflectioncorresponding to the two planes (σ_(v) and σ_(h)) that cross section thecarbon nanotube, as indicated in FIG. 8 a.

FIGS. 8 a, 8 b, and 8 c illustrate, respectively, the model of thecarbon nanotube, the corresponding band structure, and the correspondingelectron density of states.

In particular, the symmetry groups of the system illustrated in FIG. 8 aare represented by:

-   -   screw axis: rotation of α=2π/8 about z followed by a translation        of (a/2) along z;    -   plane of vertical reflection xz: σ_(v);    -   plane of horizontal reflection xy: σ_(h).

It is recalled that a symmetry group is constituted by the set of allthe operations of symmetry of the cylindrical system with which a carbonnanotube can be geometrically schematized. The term “group” is used in amathematical sense and indicates that all the operations of symmetry ofthe system are reunited so as to form a non-empty set that satisfieswell-determined mathematical conditions, which represent the formalproperties of a group (closure property, associative property, existenceof the neutral element, existence of a single inverse element).

The electron density of states (DOS) of carbon nanotubes has acontinuous energy-band structure, separated by energy gaps notaccessible to electrons, the so-called bandgaps. The structure of thesebands, their energy width, and the values of the gaps between the bandsdepend upon the characteristics of the carbon nanotube (single-wall ormulti-wall, zigzag or armchair), and upon the indices n and m.

The band structure of the carbon nanotube is obtained using the“tight-binding” model (Neil W. Ashcroft, N. David Mermin, Solid StatePhysics, Philadelphia: Saunders College (1976)), through which theenergy spectrum is obtained:

${ɛ( {m,k} )} = {{\pm \beta}\sqrt{1 + {4{\cos^{2}( {m\; \alpha} )}} + {4\; {\cos ( {m\; \alpha} )}{\cos ( \frac{ka}{2} )}}}}$

The above expression shows the dependence of the energy upon thevariables k and m and consequently indicates the possibility ofreconstructing the entire band structure of the nanotube according tothe value assumed by the variables k and m.

In the above expression:

m is the angular momentum which, in the case of the optical transitionsfor the system considered, can assume the values 0, ±1, ±2, ±3, 4;

k is the quantum number indicating the quasimomentum along the z axis,connected to rules of crystalline symmetry of the structure considered;

a is the lattice pitch of the carbon nanotube;

α is the angle of rotation, corresponding to 2π/8; and

β is the energy associated to the term of overlapping of orbitalsbelonging to adjacent atoms; this term provides an indication of theoverlap energy contribution linked to the distribution of the atomsarranged according to the crystallographic structure considered.

As highlighted in FIG. 8 b, the valence and conduction bands aresymmetrical with respect to the zero value of energy and are dividedinto non-degenerate bands, designated by A, and degenerate bands,designated by E. The degenerate energy bands are characterized by thefact that to each energy E(k) there corresponds more than one state.

Since the wavelength λ of visible light is large if compared to thelattice pitch a of the carbon nanotube, the conservation of linearmomentum implies Δk≈0, where Δk represents the difference between thevalues of the crystalline momentum k of the electron, before and afteran interband transition; the crystalline momentum is introduced into thetheory of solids, and to a first approximation can be identified withthe linear momentum of the electron.

This means that the optical transitions allowed are basically vertical;by the term “optical transitions” is meant radiative electrontransitions, i.e., ones with absorption (in excitations) or emission (inde-excitations) of radiation.

As regards the angular momentum of the electron involved in thetransition, the rules of selection depend upon the orientation of theelectrical field associated to the electromagnetic radiation incidentupon the carbon nanotube. If the incident light is linearly polarizedparallel to the axis of the carbon nanotube, then only the transitionsbetween pairs of bands so that Δm=0 are allowed.

If the light is circularly polarized in the plane orthogonal to the axisof the carbon nanotube (i.e., the direction of propagation is parallelto the axis of the carbon nanotube), only transitions between bands suchthat Δm=±1 can occur. The sign ± depends upon the phase difference (90°or else −90°) of the two components of the polarization vector.

The table of FIG. 9 presents, among other things, the transitionsallowed corresponding to the different states of polarization of theincident light. The first column of table of FIG. 9 lists the parametersof polarization. The second column of the table list the parametersassociated with linearly polarized light parallel to the axis of thecarbon nanotube. The third column of the table lists the parametersassociated with circularized polarized light orthogonal to the axis ofthe carbon nanotube.

For semiconductor carbon nanotubes like the one considered, if theenergy of the photon is greater than the gap between bands, asignificant photoconductivity due to a transition of electrons from thevalence band to the conduction band is observed.

The optical absorption spectrum and, in particular, the position of thefirst intense peak of interband transition, is markedly dependent uponthe polarization of the incident electromagnetic radiation, ashighlighted in FIG. 10, which illustrates the absorption spectrum forelectromagnetic radiation polarized in a direction parallel to and adirection perpendicular to the axis of a carbon nanotube of the typeconsidered.

Consequently, by illuminating a semiconductor carbon nanotube withappropriate radiation, it is possible to induce interband transitions ofelectrons, via photon absorption, with consequent variation of theconductivity of the carbon nanotube itself.

In particular, a light is sent onto the carbon nanotube of a wavelengthsuch that the corresponding energy of the photon may induce certaingiven electron transitions if the radiation is polarized in anappropriate direction with respect to the axis of the carbon nanotube,and may not induce transitions if it is, instead, polarized in adirection orthogonal to the previous one.

FIG. 11 illustrates, by way of non-limiting example, a possiblestructure of an optically controlled electrical-switch device using acarbon nanotube according to an embodiment of the invention.

As may be noted, the switch device, designated as a whole by 1, can beobtained in a very simple way by depositing on a substrate 2 ofmonocrystalline silicon a thin layer of silicon oxide (SiO₂) 3 and thenforming on the silicon-oxide layer 3 a planar waveguide 4, which can,for example, be obtained by simply depositing a thin strip of siliconnitride (Si₃N₄) 5 on the silicon-oxide layer 3.

The structure thus obtained is then coated with a second silicon-oxidelayer 6, and a pair of windows 7, 8 is then opened in the secondsilicon-oxide layer 6 so as to expose two portions of the strip ofsilicon nitride 5 that function as input and output ends of thewaveguide 4. In this configuration, the silicon oxide 3, 6 functions ascladding of the planar waveguide 4, while the silicon nitride 5functions as core of the planar waveguide 4. The refraction indices ofSiO₂ and Si₃N₄ are, respectively, 1.46 and 2.05, and these values of therefraction indices do not determine any restriction on the angle ofacceptance of the waveguide 4, which is defined as the maximum anglewhereby the external radiation can impinge upon the waveguide 4 so as tobe conveyed within it.

On top of the second silicon-oxide layer 6, on opposite sides of one ofthe two windows opened in the second silicon-oxide layer 6 itself, inthe example illustrated in FIG. 11 the window 7, two electrodes 9, 10are obtained, for example made of gold, which function as contacts ofthe switch device 1. In a position facing the end of the waveguide 4exposed through the window 7, there is then set a carbon nanotube 11,which is set in electrical contact with the two electrodes 9, 10.

By then sending, on the other end of the waveguide 4, exposed throughthe window 8, which is opened in the second oxide layer 6,electromagnetic radiation having a frequency and polarization, withrespect to the axis of the carbon nanotube 11, such as to induce thereininterband electron transitions, the electromagnetic radiation, afterhaving traversed the stretch of waveguide 4 set below the secondsilicon-oxide layer 6, comes out of the end of the waveguide 4 facingthe carbon nanotube 11 through the window 7, impinging upon the carbonnanotube 11.

The consequent photon absorption by the carbon nanotube 11 brings aboutan interband electron transition and a marked increase in the electricalconductivity of the carbon nanotube 11 itself; the electrical behaviorof the switch device 1 may consequently be likened to that of a closedswitch.

By sending, instead, on the first end of the waveguide 4 exposed throughthe window 8, electromagnetic radiation having polarization such as notto bring about any photon absorption by the carbon nanotube 11, nointerband electron transition is brought about, and hence no increase inits electrical conductivity; the electrical behavior of the switchdevice 1 may consequently be likened to that of an open switch.

The solution proposed provides a number of degrees of freedom on whichto act for the purposes of controlling the switch device. In fact, it ispossible to act on the operating state of the switch device either byappropriately choosing the wavelength of the incident radiation, or elseby regulating the electrical biasing of the switch device, i.e., thevoltage applied to the carbon nanotube set between the electrodes.

In fact, for given wavelengths the radiation is absorbed only if it hasa polarization parallel to the axis of the carbon nanotube; for otherwavelengths, there will be absorption only for radiation polarized in adirection orthogonal to the axis; and, finally, for yet otherwavelengths there will be no absorption in either of the two directionsof polarization.

Furthermore, the absorption spectrum of the carbon nanotube, i.e., theposition of the absorption peaks as a function of the wavelength changesif the electrical biasing of the carbon nanotube is modified, so thatthe electrical biasing may, for example, be fixed in such a way that theenergy of the incident photons of a given wavelength (excitation energy)will, for example, be smaller than the bandgap energy for theperpendicular polarization of the incident radiation and greater thanthe corresponding bandgap energy for the parallel polarization of theincident radiation.

In this case, if radiation that is polarized in a direction parallel tothe axis of the carbon nanotube impinges on the carbon nanotube, theenergy associated to the photons is greater than the bandgap energy andthe radiation is absorbed in so far as it is able to activate theinterband transitions, while if the incident radiation is polarized in adirection perpendicular to the axis of the carbon nanotube, then theenergy of the incident photons, since it is smaller than the bandgapenergy, is not able to activate any type of transition; the effect ofabsorption, in this case, is extremely reduced. The measuredphotocurrent consequently varies drastically with the two differentpolarizations of the incident light.

In this case, a tunable light source is not necessary: the wavelengthand the intensity of the light can be fixed while the electrical biasingof the carbon nanotube is modified.

The two states of polarization of the light incident upon the carbonnanotube, consequently, correspond to different values of absorption oflight by the carbon nanotube itself and, hence, to two different statesof conductivity of the carbon nanotube (high conductivity—HIGH state,low conductivity—LOW state). Once the electrical biasing of the switchdevice, the intensity of the incident light, and its wavelength (andhence the energy of the photons) are fixed, it is possible to controlthe switch device simply by rotating the polarization of the incidentlight.

FIG. 12 illustrates, by way of non-limiting example, one of the possibleuses of the optically controlled electrical-switch device according toan embodiment of the invention. In particular, FIG. 12 illustrates thecircuit diagram of a logic inverter with active load, designated as awhole by 12, in which the active load, designated by 13, is constitutedby an electrically controlled pull-up transistor with its channelconstituted by a carbon nanotube, while the pull-down transistor isconstituted by the switch device 1 according to the embodiment of theinvention previously discussed with reference to FIG. 11.

The embodiment of FIG. 12 may include an electrical-biasing device 14for biasing the carbon nanotube 11, through which it is possible to varythe electrical biasing of the carbon nanotube 11 itself and,consequently, absorption of the light incident thereon.

The switching times of the logic inverter are linked to the electrontransitions induced by the incident radiation. The absorption ofradiation brings about transition of electrons to the conduction band,with creation of electron/hole pairs and consequent increase in theconductivity of the carbon nanotube (HIGH state), while, in the absenceof absorption of radiation, there are no interband transitions (LOWstate). Consequently, the switching times are correlated to the times ofthe electron transitions, which are in the order of 10 ns.

FIG. 13 illustrates a possible implementation on silicon of the logicinverter of FIG. 12, in which parts that are the same as the ones ofFIG. 11 are indicated with the same reference numbers.

In particular, as illustrated in FIG. 13, the pull-down transistor has astructure altogether identical to that of the switch device 1illustrated in FIG. 11, in which the two electrodes 9, 10 now functionas drain and source terminals of the pull-down transistor 1 itself. Theelectrically controlled pull-up transistor 13, which functions as activeload and has its channel formed by a carbon nanotube, is obtained bydepositing, on the second silicon-oxide layer 6, a layer ofpolycrystalline silicon 15, which has the function of gate terminal ofthe pull-up transistor 13, on which a further very thin layer 16 ofsilicon oxide is subsequently formed. Then, the drain and sourceterminals 17, 18, which are also made of gold, are obtained on thesilicon-oxide layer 16 and between them a semiconductor carbon nanotube19, which functions as channel, is then deposited, or grown, accordingto the used technique. In particular, the drain terminal 9 of thepull-down transistor 1 and the source terminal 18 of the pull-uptransistor 13 are made integral with respect to one another, so as todefine the output terminal of the inverter, while the gate terminal 15of the pull-up transistor 13 is connected to the drain terminal 17 ofthe pull-up transistor 13 itself in a way known and not illustrated.

FIG. 14 shows, by way of non-limiting example, a different possible useof the switch device 1 according to an embodiment of the invention. Inparticular, FIG. 14 illustrates a block diagram of a system for theactivation of an electric circuit by means of an optical signal withcontrol of polarization.

In detail, as illustrated in FIG. 14, an optical signal coming, forexample, from a laser light source 20, is supplied at input to anelectrical-switch device 1 according to embodiments of the invention,which issues a command for activation of a circuit 21 set downstream,according to the polarization of the optical signal supplied at inputthereto.

In particular, the polarization of the optical signal supplied by thelaser light source 20 can be controlled by means of a control circuit 22of the type illustrated in FIG. 15. This is made up of a light polarizer23, which linearly polarizes the light coming from the laser lightsource 20, and by a Pockels cell 24, which is set downstream of thelight polarizer 23 and is capable of rotating the direction ofpolarization of the light coming out of the light polarizer 23, viaapplication of an appropriate external voltage V, by causing it to varybetween the two states parallel and perpendicular to the axis of thecarbon nanotube of the electrical switch 1 according to an embodiment ofthe invention.

As is known, a Pockels cell is a cell that exploits the so-calledPockels effect and is basically formed by an appropriate crystal, towhich, by applying a potential difference, a variation of the index ofrefraction along an axis is induced. This, in turn, induces abirefringence effect proportional to the applied voltage, thus creatinga system capable of varying the plane of polarization of the light.

From an examination of the characteristics of the optically controlledelectrical-switch device obtained according to embodiments of thepresent invention, the advantages that it makes possible are evident.

In particular, by exploiting the property of optical dichroism of carbonnanotubes, the present invention makes available an electro-opticalswitch device of nanometric dimensions which is driven by acting, ratherthan upon the presence/absence of light, as occurs in electro-opticalswitch devices according to the known art, but upon the polarization ofthe electromagnetic radiation at input.

Furthermore, embodiments of the present invention enable anelectro-optical switch to be obtained having a structure simpler thanthat of electro-optical switch devices according to the known art. Thisfunction, in fact, is usually obtained using the diagram illustrated inFIG. 16, i.e., arranging in cascaded fashion: a first light polarizerfor linearly polarizing the input light in a given direction; a Pockelscell, for rotating the direction of polarization of the light coming outof the light polarizer via the application of an appropriate externalvoltage; and a second light polarizer for linearly polarizing the lightcoming out of the Pockels cell in a direction perpendicular to that ofthe first polarizer. By acting appropriately upon the voltage applied tothe Pockels cell, it is possible to cause the electromagnetic radiationto come out or otherwise from the second polarizer. By detecting thepresence or absence of the electromagnetic radiation at output from thesecond polarizer, for example via an appropriate optical detectorcircuit, it is possible to control activation or deactivation of acircuit set downstream of the electro-optical switch device.

Using, instead, the switch device according to embodiments of theinvention, it is possible to do without the second polarizer, and thispossibility becomes particularly important for construction ofnanometric electro-optical devices, in so far as the absence of a secondpolarizer reduces the overall dimensions considerably.

Finally, it is clear that modifications and variations may be made tothe optically controlled electrical-switch device described andillustrated herein, without thereby departing from the scope of thepresent invention, as defined in the appended claims.

The described embodiments of switch device 1 and systems including theswitch device may be utilized in a variety of different types ofelectronic systems, such as computer systems and memory systems.

1-2. (canceled)
 3. An electrical-switch system, comprising: an opticallycontrolled electrical-switch device comprising, a firstcurrent-conduction terminal, a second current-conduction terminal, andat least one carbon nanotube connected between the first and secondcurrent-conduction terminals; an electromagnetic radiation generatingdevice for generating and directing electromagnetic radiations onto eachcarbon nanotube; and a polarization varying device for varying apolarization of the electromagnetic radiation emitted by theelectromagnetic radiation generating device to control the electricalconductivity of each carbon nanotube.
 4. The electrical-switch systemaccording to claim 3, wherein the polarization varying device varies thepolarization of said electromagnetic radiation in response to a controlsignal of an electrical type.
 5. The electrical-switch system accordingto claim 4, wherein the polarization varying device supplies at anoutput electromagnetic radiation having a first polarization withrespect to the axis of each nanotube to which there corresponds, in agiven condition of electrical biasing of the carbon nanotube and for agiven wavelength of the incident electromagnetic radiation, a highelectrical conductivity of the carbon nanotube, and a secondpolarization with respect to the axis of each nanotube, substantiallyorthogonal to the first polarization, to which there corresponds areduced electrical conductivity of the carbon nanotube.
 6. Theelectrical-switch system according to claim 3, wherein the polarizationvarying device comprises a polarizer and a Pockels cell arranged incascaded fashion.
 7. The electrical-switch system according to claim 3,further comprising an electrical-biasing device for biasing each carbonnanotube, designed to vary each nanotube's conditions of absorption ofthe incident electromagnetic radiation. 8-11. (canceled)
 12. Afabrication process of an optically controlled electrical-switch device,of the method comprising: forming a waveguide on a substrate ofinsulating material; coating at least the waveguide with a layer ofinsulating material; removing a portion of the layer of insulatingmaterial so as to expose the underlying portion of the waveguide;forming a first electrode and a second electrode of electricallyconductive material on opposite sides of the exposed portion of thewaveguide, and forming a carbon nanotube between Lithe first and secondelectrodes in a position facing the exposed portion of the waveguide.13. (canceled)
 14. A switch device, comprising: first and secondterminals; a nanotube structured between the first and second terminals,the nanotube being operable as a function of a polarization ofelectromagnetic radiation incident upon the nanotube to control acurrent flowing through the nanotube; and a waveguide structured toreceive electromagnetic radiation having a given polarization and toguide this electromagnetic radiation to the nanotube to thereby controlcurrent through the nanotube as a function of the polarization of theelectromagnetic radiation.
 15. The switch device of claim 14 wherein theelectromagnetic radiation has a first polarization that corresponds to afirst logical state in the switch device.
 16. The switch device of claim15 wherein the electromagnetic radiation has a second polarization thatcorresponds to a second logical state in the switch device.
 17. Theswitch device of claim 16 wherein the first polarization is orthogonalto the second polarization.
 18. The switch device of claim 14 furtherincluding an electrical bias circuit that applies a bias voltage to thefirst and second terminals, the bias voltage operable to adjust awavelength of electromagnetic radiation that is absorbed by thenanotube.
 19. The switch device of claim 18 wherein for a given value ofthe bias voltage the nanotube is operable to present a high conductivitystate for electromagnetic radiation having a first polarization and alow conductivity state for electromagnetic radiation having a secondpolarization that is orthogonal to the first polarization.
 20. Theswitch device of claim 14 further comprising a load element coupled toone of the first and second terminals to thereby form a logic invertercircuit.
 21. An electronic circuit, comprising: an optical generationcircuit operable to generate an optical signal; a polarization circuitpositioned adjacent the optical generation circuit and operable topolarize the optical signal to generate a polarized optical signal; anda switch device positioned adjacent the polarization circuit to receivethe polarized optical signal, the switch device including, firstterminal and second terminals; and a nanotube structured between thefirst and second terminals, the nanotube being operable as a function ofthe polarization of the polarized optical signal to control a currentflowing through the nanotube between the first and second terminals. 22.The electronic circuit of claim 21 wherein the optical generationcircuit comprises a laser.
 23. The electronic circuit of claim 21wherein the polarization circuit comprises: a light polarizer adapted toreceive incident light; and a Pockels cell positioned adjacent thepolarizer to receive polarized light from the polarizer.
 24. Anelectronic system, comprising: a switch device including, first terminaland second terminals; a nanotube structured between the first and secondterminals, the nanotube being operable as a function of a polarizationof electromagnetic radiation to which the nanotube is exposed to controla current flowing through the nanotube; a waveguide structured toreceive electromagnetic radiation having a polarization and to guidethis electromagnetic radiation to the nanotube; and electronic circuitrycoupled to the waveguide, the electronic circuitry operable to provideelectromagnetic radiation to the waveguide and to control thepolarization of this electromagnetic radiation.
 25. The electronicsystem of claim 24 wherein the electronic circuitry comprises computercircuitry. 26-29. (canceled)
 30. The electronic system of claim 24further comprising additional electronic circuitry coupled to the switchdevice and operable to sense a current flowing through the nanotube. 31.The electronic system of claim 24 wherein the electronic circuitryprovides electromagnetic radiation to the waveguide having either afirst polarization or a second polarization.